A builder is a construction and installation company with which a real estate development company signs a construction contract through bidding. It can be the general contractor or the construction unit.
The real estate company can't be the contractor, because according to the national building regulations, Party A and Party B can't be the same company, which lacks fairness and can't guarantee the construction quality of the project. But there can be an engineering management department to control the construction progress, quality and safety of development projects.
The real estate company is the seller, and a sales department is set up to be responsible for the planning and implementation of sales.